In modern cellular radio systems, the radio network has a strict control on the behaviour of the terminal. Uplink transmission parameters like frequency, timing, and power are regulated via downlink control signalling from the base station to the terminal. This is also true for long term evolution (LTE) based cellular wireless communication networks.
In the uplink, a physical layer is based on Single Carrier-Frequency Division Multiple Access (SC-FDMA), which is also referred to as pre-coded Orthogonal Frequency-Division Multiplexing (OFDM). A cyclic prefix is used before each SC-FDMA symbol in order to combat channel delay spread and propagation delay. The cyclic prefix is prefixing of a symbol with a repetition in a wireless communication signal. The communication system uses the cyclic prefix for providing a guard interval to handle delays and provide a support for Fast Fourier Transform (FFT) processing of the signals.
In order for a base station (e.g. called eNodeB in LTE based networks) to control a terminal (also called user equipment, abbreviated to UE) or other equipment connecting to the network, measurements on an uplink signal are required. The determination of timing misalignment requires a comparison of the timing of a received signal with a reference clock in eNodeB. Timing misalignment is caused by unknown propagation delay and the mutual drift between the clocks in the eNodeB and in the UE.
Uplink measurements are rather straightforward when the UE has established a duplex connection with the eNodeB. In that case, uplink signals are present for the measurements, whereas downlink signals can carry the control signalling to adjust the UE parameters. However, when the UE is not connected but is in standby, it only listens to the downlink control signal periodically. Thus, there is no uplink signal for the eNodeB to measure. Before connection establishment, the UE has to carry out a random access (RA) procedure. This is initiated by the UE transmitting a random access burst through a radio interface to the eNodeB to request channel assignments. This random access burst is performed on a random access channel (RACH).
A physical random access channel (PRACH) is provided for the UE to request access to the network. This means that random access bursts must be detected with good confidence and, when detected, used for propagation delay estimation. The used access burst (AB) contains a preamble with a specific bit sequence that has good auto-correlation properties.
In for example the 3GPP standard for LTE [3GPP TS 36.211 v10], the PRACH is arranged to comprise up to 64 different preambles which the UE can select among. These sequences are constructed by a number of base sequences and cyclic shifts of these. Here, the choice of the size of the cyclic shift is depending on the delay spread of the channel and the maximum round trip time of the current cell. For a small cell, this cyclic shift can be configured to a small number such that only one base sequence is needed for construction of all 64 preambles.
FIG. 3 shows the PRACH signalling scheme between UE and eNodeB. The signalling starts with the UE sending a preamble within a predefined time window. The eNodeB then needs to detect the incoming PRACH preamble and send a Random Access Response (RAR) containing an identifier of the used preamble within a given timeframe. The identifier shows the detected preamble (out of the 64 possible) and the time-frequency slot in which the preamble was detected. This RAR also contains timing alignment (TA) instructions, an initial uplink resource grant and an assignment of a Cell Radio Network Temporary Identifier (C-RNTI). The UE then answers with a so-called “Step 3 message” which is used for early contention resolution, i.e. for instance to resolve any collisions related to several UEs using identical signatures when initiating the PRACH procedure.
Lately there has been an increased interest in smaller cell deployment for instance in home or office environment. However, the PRACH procedure for LTE is typically designed (and standardized) for macro cell deployment, which gives room for simplifications for small cell deployment.
The PRACH detection has a potential of being very demanding when it comes to hardware requirement for eNodeB. A typical PRACH detector (although there are alternative solutions) may for instance contain large buffers, a DFT of size 24576, several inverse DFTs of size 1024, and matched filters. This type of LTE PRACH detector is unnecessarily complicated for small cells. Hence there is a need for method and apparatus to reduce PRACH hardware load for an eNodeB in a small cell.